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technical review and discussion

Measurement of Gas evolution from PunB Bonded sand as a function of temperature G. Samuels and C. Beckermann; Dept. of Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA, USA

Reviewer: The main drawback of the experimental tech- nique is that the collected gas is not necessarily the gas trans- porting through the core under casting conditions. Gas-metal physical (dissolution) and chemical reactions should affect what transports and it would be useful to study this interac- tion in a controlled experiment.

Authors: Additional experiments may be needed to study the effect of gas-metal interactions. The present study fo- cuses on binder gas evolution away from the metal-mold interface. Unlike in previous experiments, the evolved gas in the present volume measurement apparatus stays in contact with both the already evolved gas and the yet unreacted binder. This is quite close to the situation in- side of a mold, away from the metal-mold interface. It is true that the binder gas is transported in the mold and its composition continually changes. But the atmosphere in the present measurement apparatus is evolved binder gas, not some inert gas.

Reviewer: The claim that the rate dependence is weak is unsubstantiated. Metal-mold interface heating rates are at least an order of magnitude higher than those achieved in the experiment. Even at the achieved rates, the overall shift of the decomposition curve to the right by about 50 degrees in Figure 3 is in fact not insignificant. The trend will continue as the heating rates begin approaching cast- ing conditions.

Authors: Yes, heating rates at the metal-mold interface are at least an order of magnitude higher. As is stated in the paper, the heating rates in the present experiments are encountered during casting at distances from the metal-mold interface greater than about 1.3 cm (0.5 in). The statement that the rate dependency is not strong up to 100o

C/min is substantiated by the present TGA measure-

ments. The shifts in Fig. 3 are explained in the paper as follows: “More likely, the delays are simply caused by greater non-isothermality of the bonded sand samples for higher heating rates. For high heating rates, the center of the bonded sand samples can be expected to be some- what cooler than the surface. This implies that the lower heating rate results better reflect the ‘true’ decomposition behavior at a given temperature.” In other words, we be- lieve that the shift to the right at higher heating rates is simply due to non-isothermality of the sample in the TGA apparatus. Our studies with higher TGA gas flow rates (see Samuels’ thesis) also back up this statement.

International Journal of Metalcasting/Spring 2012

Furthermore, we say in the Conclusions: “Since the present TGA measurements reveal that the binder gas mass evolu- tion is not a strong function of the heating rate up to 100°C/ min (180°F/min), it may be concluded that the present data can be used to describe the gas evolution behavior at any distance from the mold-metal interface. Such a conclusion may be erroneous and additional measurements at very high heating rates, corresponding to locations very near the mold-metal interface, are needed to investigate the rate dependency.”

Reviewer: Polynomial fits used in Table 3, while useful, are not exactly appropriate to the context. A first order kinetic fit (multi-rate) has the mentioned “shift-up” effect built into it, was used by Lytle and McKinley to parameterize TGA of PUCB, and could be used here.

Authors: The present polynomial fits are appropriate up to about 100o

C/min, when the heating rate effect is small. While

we did perform TGA measurements at multiple heating rates, and could have done a “kinetic” fit of those data, it would have been inappropriate due to the closeness of these TGA data (considering the scatter even at a single heating rate) and the lack of data at higher heating rates. We believe that the “shift-up” effect seen in the present TGA measurements is simply due to sample non-isothermality, not any chemical kinetics. We did not perform gas volume measurements at different heating rates (only at 2o

C/min), so a “kinetic” fit of the molecular weight data is not possible.

Reviewer: A multi-rate 1st order Arrhenius fit would allow comparison of binders. A claim is made that the two (PUCB and PUNB) are “similar”. Judging from the TGA in [20, 21], this claim is incorrect. The decomposition range of PUCB with 150 deg/min heating is ~77-527 C and the residue frac- tion is 28%. The data in Figure 3 for PUNB suggest a range of ~200-700 C and a residue fraction of 15%. Further the decomposition rate of PUNB looks bi-modal, while PUCB decomposition rate is not.

Authors: The paper introduction states that PUCB and PUNB can be “reasonably compared”. However, the paper does not use the PUCB data for much other than to show that at high temperatures, the previously measured gas mo- lecular weights for PUCB and PUNB are very close to each other. A more detailed comparison with PUCB is outside the scope of the present study. The residue fraction for PUCB of 28% in references 20 and 21 is likely inaccurate and is in reality probably closer to our measured value for PUNB of 18% (not 15%). We state in the paper that “It should be noted that applying the correction changes the fraction of original binder mass remaining by as much as 10% at the maximum decomposition temperature.” This refers to the correction of the bias in the TGA machine. The details of this correction are provided in Samuels’ thesis. In references 20 and 21, the bias was not corrected.


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